The fabrication of electronic microcircuits requires the use of substrates, heatsinks, electrodes, leads, connectors, packaging structures and other components capable of dissipating the heat generated by the active parts of the microcircuit or by the soldering, brazing or glass-sealing process. Moreover, those components that are in direct contact with the active microcircuit sections must have a coefficient of thermal expansion ("CTE") compatible with gallium arsenide, silicon, germanium or any material used in the fabrication of the microcircuit.
Materials such as copper, silver, gold and aluminum which exhibit high coefficient of thermal conductivity tends also to have CTEs much higher than materials such as gallium arsenide alumina or polysilicon which are used in the implementation microcircuit elements or their enclosures.
As disclosed in U.S. Pat. No. 4,680,618 Kuroda et al., it has been found convenient to use composites of copper and other denser metals such as tungsten or molybdenum in the fabrication of heatsinks, substrates and other heat-dissipating elements of microcircuits. The proportions of the metals in the composite are designed to match the CTE of the material used in the fabrication of the active circuit component.
The CTE of a metal is defined as the ratio of the change in length per degree Celsius to the length. It is usually given as an average value over a range of temperatures. Metal used in electrical conductors such as aluminum, copper, silver and gold that have a low electrical resistivity also exhibit high coefficients of thermal conductivity. The coefficient K of thermal conductivity of a material is defined as the time rate of heat transfer through unit thickness, across unit area, for a unit difference in temperature or K=WL/A T where W=watts, L=thickness in meters, a=area in square meters, and T=temperature difference in .degree.K or .degree.C. For copper, K is equal to 388. For silver, K is equal to 419. However, these metals exhibit an average CTE in excess of 15.times.10.sup.-6 /.degree.C. By contrast, material such as gallium arsenide and silicon, that are commonly used in the manufacture of microcircuit chips have an average CTE of less than 7.times.10.sup.-6 /.degree.C. Thus, while material of high electrical and thermal conductivity are favored in the fabrication of heat-dissipating electric elements, they must be blended with conductive material exhibiting a much lower average CTE in order to create a composite whose thermal expansion characteristic comes as close as practically possible to that of gallium arsenide, silicon and other micro-chip materials. Tungsten and molybdenum with average CTE of 4.6.times.10.sup.-6 /.degree.C. and 6.times.10.sup.-6 /.degree.C. and coefficient of thermal conductivity of 160 and 146 respectively are favored.
However, while copper, aluminum, and silver have specific gravities of less than 9, and melting point of less than 1,100.degree. C., tungsten and molybdenum have specific gravities of 19.3 and 10.2, and melting points of 3,370.degree. C. and 2,630.degree. C. respectively.
Due to the large differences in the specific gravities and melting-points, and lack of mutual solubility of metals such as copper and tungsten, for example, it is difficult to form composites of those two metals that exhibit a reliable degree of homogeneity using conventional melting processes.
As disclosed in U.S. Pat. No. 5,086,333 Osada et al., it has been found more practical to press and sinter a powder of the most dense materials, e.g., tungsten, to form a porous compact, then to infiltrate the compact with molten copper or another lighter material. A slab of the resulting material can then be cut and machined to form heatsinks, connectors, substrates and other heat-dissipating elements.
The heat-dissipating base upon which micro-chips are mounted must also be attached to packaging or frame member usually made of ceramic or other material having a different CTE than the semiconductor material of the micro-chip and of the heat-dissipating base. Thermal stress between the heat-dissipating base and the frame member may cause cracks or camber after joining operation in the latter stage. The problem has been addressed in the prior art by using a intermediate heat-dissipating member whose composition and CTE continuously vary from one contact to the other as disclosed in U.S. Pat. No. 3,097,329 Siemens.
Another approach disclosed in U.S. Pat. No. 4,427,993 Fichot et al. consists of embedding a lattice of CTE-modifying material into one of the contact surfaces of the heat-dissipating element.
Both of these approaches are, complex and onerous and do not allow a precise control of the CTEs at one or both interfacing areas of the heat-dissipating member.
The costs of metals such as tungsten and molybdenum are relatively high compared to the costs of copper, aluminum and other more abundant metals. Heat-dissipating components made of composites in which a costly metal such as tungsten is used for CTE-matching purpose tend to be expensive. As the power ratings of micro-electronic modules increase, bigger heat-dissipating substrates are required. The cost associated with the substrates being dictated by the cost of their base metals remains inflexibly high while the costs of the microcircuit can be controlled and even reduced through the use of new technological improvements. The problem of substrate-related costs is particularly acute in microwave power devices where large and elaborate heat-sinks must be used. In many cases, the costs of the heat-dissipating component represent a large percentage of the total device. Accordingly, there is a need for a more economical way to construct heat-dissipating substrates for high-power micro-electric modules.
The instant invention results from an attempt to devise a simpler, more practical and more economical process to manufacture such heat-dissipating components using powder and other metallurgy techniques. The invention is based in part on the techniques and processes disclosed in the parent application, Ser. No. 08/064,255 which issued as U.S. Pat. No. 5,413,751 dated May 9, 1995, which application and patent are hereby incorporated in this Specification by reference.